The invention relates generally to the production of modified polymers of high molecular weight poly(lactic acid) useful for waste disposal or for agricultural purposes. More particularly, the invention relates to producing high molecular weight poly(lactic acid) by coupling lower molecular weight version of the polymer (molecular weight of about 2,000-15,000) by various coupling agents. These increased molecular weight products have improved mechanical properties while still degrading in an environmentally safe manner. The modified polymers and polymer blends are also usable for garbage bags or other waste disposal purposes and can be incinerated since upon burning only environmentally safe materials are produced. It is now recognized that many plastic materials useful for packaging as well as waste disposal present serious environmental problems because they either do not degrade in landfills or produce toxic components upon incineration. More attention has been devoted to environmentally safe plastic materials, particularly since Congressional hearings have determined that over 135 thousand metric tons per year of plastic is discarded at sea alone with some 639,000 plastic containers and bags tossed into the ocean every day. In addition to this intolerable situation, the amount of nondegradable plastic materials used for packaging as well as garbage disposal has caused landfill areas (previously thought to be entirely adequate for urban disposal sites) to become filled and unusable. Waste disposal has become a very serious problem in this country as well as world-wide.
Accordingly, there is a need to provide plastic materials suitable for packaging and waste disposal which at the same time will also be degradable to products which are environmentally safe. However, it is also generally recognized that higher molecular weights (above at least 25,000) are needed for poly(lactic acids) to have good physical properties.
In the area of medical applications, the polymers of lactic acid and glycolic acid achieved their first commercial success as fiber materials used for reabsorbable sutures. These were first announced by Davis & Geck in 1969 and since then have significantly displaced collagen as resorbable sutures. The polymers of lactic acid are well suited for other prosthetic devices because they are hydrolyrically degradable, biocompatible, and also they are thermoplastics suitable for the extrusion, molding, etc., of cross-sectional designs.
Kulkarni first proposed the use of poly(d,l-lactic acid) as a resorbable prosthetic device. Working in the U.S. Army's Biomedical Laboratory, Kulkarni recognized the possibilities of this material for repair and reconstruction of traumatic wounds. The materials replace steel plates, pins, etc., allowing the bone to remodel its stresses naturally as the polyesters gradually resorb. A second operation to remove the metal device is obviated as is the need for a second surgical procedure with the use of conventional, autogenous grafts. Subsequent work at the U.S. Army Institute of Dental Research again demonstrated biodegradability and biocompatibility, in this case of implants or more sizable cross-sections such as films, slabs, and pins that were not oriented, plasticized (as with sutures) and for copolymer compositions that also included d,l-lactic acid. The polymers of lactic acid then act as a temporary fastnet and scaffold for hard or soft tissue, providing strength until natural healing occurs.
Since this beginning in the medical field, a number of research organizations have worked extensively on resorbable prosthetic devices. Mandibular fractures, long bone internal fixation, osteogenesis, nerve regeneration, vascular prostheses, and Achilles tendon repair are just a few examples illustrating the variety of devices. A number of these devices are just now beginning to emerge from preclinical and clinical trials.
There are probably only a few ten thousands pounds of poly(lactic acid) marketable for prosthetic devices. Based on the small weights involved, the expense is quite large to produce premium purity poly(lactic acid) products using current methods of condensation polymerization of free acids and catalytic, ring opening polymerization.
The use of the polymer in these medical devices may draw attention to these degradable plastics for larger uses. Assuming 400,000 metric tons of pesticide are used per year in the U.S., a sizable use of poly(lactic acid) as controlled release coatings is possible. This application would require 2-3 times the tonnage of the formulation agent, poly(lactic acid). Success in this market may depend on government regulations to prevent pesticide runoff or producers successfully pursuing controlled release advantages. With only a modest market penetration the use for pesticide and diverting agents could reach a few tens of million pounds consumption per year.
The benefits to the corn grower could be substantial. An acre of land devoted to corn can be expected to yield 100 to 140 bushels, depending on weather and agricultural practices. The yield can be expressed in terms of lactic acid that could be generated from corn starch with a good fermentation system. Because the fermentation of glucose to lactic acid involves no loss of carbon dioxide, lactic acid production could be 4,000 to 5,000 pounds per acre.
This output should be contrasted with the potential for making ethanol. About 2.4 gallons of ethanol per bushel is the limit on ethanol production because about half of the weight of glucose is lost as carbon dioxide. Therefore, about 240 to 340 gallons of ethanol can be made from the corn available from an acre of land. Processes that produce carbon dioxide as a by-product are also less desirable because of increasing concern about its accumulation in the atmosphere as a "Greenhouse Gas."
At a selling price of $1.50 per gallon, an acre of land would yield $360 to $500 of ethanol. The same acre would yield $1,000 to $1,250 of lactic acid with a fermentation of comparable difficulty and by-product feeds of comparable value. The impact of lactic acid production on corn grower prosperity would, however, depend on market size and the development of large-scale markets. The fuel market for ethanol is already developed but is shaky from the viewpoint of near-term economic driving force. Building a lactic acid commercial foundation for future prosperity could have substantial returns for corn growers.
Recently, it has been determined that high carbohydrate food waste presently produced in the United States as cheese whey permeate and in conjunction with potato processing facilities is convertible in an environmentally benign process to provide a feed stream for lactic acid. Lactic acid is desirable because it is a naturally occurring compound which degrades to environmentally safe products. In addition, it has been discovered that oligomers of poly(lactic acid) are useful as plant growth promoters, see U.S. Pat. No. 4,813,997 issued to Kinnersley et. al. With the discovery of the conversion of high carbohydrate food waste to feedstocks for lactic acid, it has become feasible through the present invention to formulate various copolymers and blends of poly(lactic acid) for a wide range of agricultural and packaging uses. These applications meet all the objectives set forth above and provide environmentally safe materials to replace presently used plastics that are difficult to dispose of in a safe manner.
Accordingly, it is an object of the invention to provide a novel degradable high molecular weight polymer of modified poly(lactic acid) polymers or blends thereof.
Another object of the invention is to provide an improved poly(lactic acid) polymer and copolymer of high molecular weight by coupling lower molecular weight units using difunctional coupling agents.
It is a further object of the invention to provide a novel high molecular weight, modified poly(lactic acid) polymer using a hydroxyl group or carboxylic acid terminated poly(lactic acid) polymer and using a coupling agent selected from the class consisting of di-isocyanates, bis-epoxides, bis-oxazolines and bis-ortho esters.
A still further object of the invention is to provide an improved high molecular weight poly(lactic acid) polymer comprising a co-polymer of the high molecular weight poly(lactic acid) and a modifying monomer selected from the class consisting of p-dioxanone present in an amount up to about 20% by weight, 1,5 dioxepan-2-one present in an amount up to about 20% by weight, and 1,4 oxathialan-2-one, 4,4 dioxide present in an amount up to about 20% by weight, or mixtures thereof, the modifying monomer being present in an amount not greater than about 20% by weight.
An additional object of the invention is to provide a novel method and new poly(lactic acid) polymer having terminal mercaptan groups readily oxidizable to di-sulfides.
The invention consists of certain novel features and a combination of parts hereinafter fully described, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.